In recent years, surface-treated steel sheets produced by imparting rust-preventive properties to base material steel sheets, in particular, hot-dip galvanized steel sheets and hot-dip galvannealed steel sheets, have been widely used in the fields of automobiles, household appliances, building materials, and the like. Furthermore, from the standpoint of improvement in fuel consumption of automobiles and in crashworthiness of automobiles, there has been an increased demand to decrease thickness by strengthening the materials for automobile bodies and to decrease the weight of and increase the strength of automobile bodies. For that purpose, application of high-strength steel sheets to automobiles has been promoted.
In general, a hot-dip galvanized steel sheet is produced by a method in which a thin steel sheet obtained by hot rolling or cold rolling a slab is used as a base material, and the base material steel sheet is subjected to recrystallization annealing and a hot-dip galvanizing treatment in an annealing furnace in a continuous hot-dip galvanizing line (hereinafter, referred to as “CGL”). When a hot-dip galvannealed steel sheet is produced, after the hot-dip galvanizing treatment, a galvannealing treatment is further carried out.
Examples of the heating furnace type of an annealing furnace in a CGL include a DFF type (direct fired furnace type), a NOF type (non-oxidizing furnace type), and an all radiant tube type. In recent years, CGLs equipped with all radiant tube type heating furnaces have been increasingly constructed because of ease of operation, less likely occurrence of pickup, and the like, which makes it possible to produce high-quality coated steel sheets at low cost. However, unlike the DFF type (direct fired furnace type) or the NOF type (non-oxidizing furnace type), since an oxidizing step is not performed immediately before annealing in the all radiant tube type heating furnace, the all radiant tube type heating furnace is disadvantageous in terms of securing coatability regarding steel sheets containing easily oxidizable elements, such as Si and Mn.
As the method for producing a hot-dip coated steel sheet including, as a base material, a high-strength steel sheet containing large amounts of Si and Mn, Japanese Unexamined Patent Application Publication No. 2004-323970 and Japanese Unexamined Patent Application Publication No. 2004-315960 each disclose a technique in which, by increasing the dew point by specifying the heating temperature in a reducing furnace using a relational expression with a water vapor partial pressure, the surface layer of the base material is internally oxidized. However, since the area where the dew point is controlled is assumed to be the entire inside of the furnace, it is difficult to control the dew point, and stable operation is difficult. Furthermore, when a hot-dip galvannealed steel sheet is produced with unstable control of dew point, there is a variation in the distribution of internal oxides formed in the substrate steel sheet, and there is a concern that defects, such as uneven wettability of coating and uneven galvannealing, may occur in the longitudinal direction and in the width direction of the steel sheet.
Furthermore, Japanese Unexamined Patent Application Publication No. 2006-233333 discloses a technique in which by specifying not only H2O and O2, which are oxidizing gases, but also the CO2 concentration at the same time, the surface layer of the base material immediately before coating is internally oxidized, and external oxidation is suppressed, thereby improving coating appearance. However, in JP '333, as in JP '970 and JP '960, because of the presence of internal oxides, fractures easily occur during working, and resistance to peeling of coating is degraded. Degradation in corrosion resistance is also observed. Regarding CO2, there is a concern that contamination may occur in the furnace or carburization may occur in the surface of the steel sheet, resulting in a change in mechanical properties.
Furthermore, recently, high-strength hot-dip galvanized steel sheets and high-strength hot-dip galvannealed steel sheets have been increasingly applied to spots that are difficult to work, and resistance to peeling of coating during high-level work has been regarded as important. Specifically, when a coated steel sheet is subjected to bending work with a bending angle exceeding 90° to be bent at an acute angle or a steel sheet is subjected to working because of an applied impact, it is required to suppress peeling of coating at the working spot.
To satisfy such properties, it is not only required to ensure a desired texture of a steel sheet by adding a large amount of Si to the steel, but it is also required to more highly control the texture and structure of a surface layer of a substrate steel sheet directly below the coating layer, from which fractures and the like during high-level work may originate. However, such control is difficult with conventional techniques. It has not been possible to produce a hot-dip galvanized steel sheet having excellent resistance to peeling of coating during high-level work, using a Si-containing high-strength steel sheet as a base material in a CGL equipped with an all radiant tube type heating furnace as an annealing furnace.
It could therefore be helpful to provide a high-strength hot-dip galvanized steel sheet including, as a base material, a steel sheet containing Si and Mn and having excellent coating appearance, corrosion resistance, and resistance to peeling of coating during high-level work; and a method for producing the same.